Electrostatic trap

Abstract

An electrostatic trap such as an orbitrap is disclosed, with an electrode structure. An electrostatic trapping field of the form U′(r, ϕ, z) is generated to trap ions within the trap so that they undergo isochronous oscillations. The trapping field U′(r, ϕ, z) is the result of a perturbation W to an ideal field U(r, ϕ, z) which, for example, is hyperlogarithmic in the case of an orbitrap. The perturbation W may be introduced in various ways, such as by distorting the geometry of the trap so that it no longer follows an equipotential of the ideal field U(r, ϕ, z), or by adding a distortion field (either electric or magnetic). The magnitude of the perturbation is such that at least some of the trapped ions have an absolute phase spread of more than zero but less than 2π radians over an ion detection period Tm.

The invention claimed is: 
     
       1. A method of trapping ions of a mass to charge ratio m/q in an electrostatic trap having an electrode assembly, comprising:
 a. trapping the ions in a trapping volume of an external storage device that comprises a curved multipole trap; 
 b. applying a voltage pulse to an exit electrode of the external storage device so as to eject the ions into and through a deflection lens arrangement and into the electrostatic trap, wherein a magnitude of the pulse and a voltage applied to the deflection lens arrangement are chosen such that the ions arrive at an entrance to the electrostatic trap as a focused packet of ions; 
 c. applying a substantially electrostatic trapping potential to at least a part of the electrode assembly, so as to generate an electrostatic trapping field within the trap, for trapping the ions of the mass to charge ratio, m/q, in a volume, V, of the electrostatic trap such that they undergo multiple isochronous oscillations along a longitudinal axis, z, of the electrostatic trap; and 
 d. causing a perturbation in the electrostatic trapping field which results in at least a portion of the packet of ions to undergo a separation in oscillation phase of no more than 2n radians over a measurement time period, T m , the perturbation arising from at least one of the following: a distortion of the electrostatic trap geometry, a distortion of a part of the trapping potential, and application of an additional distortion potential to at least one part of the electrode assembly. 
 
     
     
       2. The method of  claim 1 , wherein the perturbation has an extent such that an average rate of change, dτ/dA z , of an oscillation period, τ, of oscillation of the ions within volume, V, parallel to the longitudinal axis, z, of the electrostatic trap with respect to an amplitude, A z , of said oscillation is positive such that an increasing amplitude of oscillation causes an increase in ion oscillation period. 
     
     
       3. The method of  claim 1 , wherein the perturbed trapping field is of the form U′(r,ϕ,z)=U(r,ϕ,z)+W, where U(r,ϕ,z) is an ideal trapping potential and W is the perturbation thereto, and wherein the step of distorting the geometry of the electrostatic trap comprises distorting the shape of at least a part of the electrode arrangement such that it deviates from an equipotential of the ideal potential U(r,ϕ,z) by an amount sufficient to impart an n th  order perturbation to the ideal potential U(r,ϕ,z), where n≥2. 
     
     
       4. The method of  claim 3 , wherein the perturbation in the electrostatic trapping field is at least partially caused by a distortion in the shape of at least a part of the electrode arrangement such that it deviates from the said equipotential of the ideal potential U(r,ϕ,z) by an amount sufficient to introduce a negative, fourth order term into the ideal expression U(r,ϕ,z). 
     
     
       5. The method of  claim 1 , wherein the electrostatic trap comprises a plurality of trapping electrodes to generate the electrostatic trapping field and at least one distortion electrode, the method further comprising applying a voltage to the distortion electrode to add a perturbation component to the electrostatic trapping field so as to create at least a part of said perturbation in the electrostatic trapping field. 
     
     
       6. The method of  claim 1 , wherein the electrostatic trap comprises first and second electrode structures defining therebetween the said trapping volume, V, wherein surfaces of the first and second electrode structures generally follow equipotential surfaces of an ideal trapping field, and wherein the geometry of the electrostatic trap is distorted by stretching or shifting one or both of the first and second electrode structures relative to the ideal trapping field equipotentials so as to introduce the said geometric distortion that results in said ion phase separation. 
     
     
       7. The method of  claim 1 , wherein the perturbation arises from at least an application of an additional distortion potential to at least one part of the electrode assembly, wherein the application of the additional distortion potential comprises causing a magnetic field to pass through the trapping volume, V. 
     
     
       8. A mass spectrometry method comprising:
 a. trapping ions in a trapping volume of an external storage device that comprises a curved multipole trap; 
 b. applying a voltage pulse to an exit electrode of the external storage device so as to eject the ions into and through a deflection lens arrangement and into an electrostatic trap having an electrode assembly, wherein a magnitude of the pulse and of a voltage applied to the deflection lens arrangement are chosen such that the ions of each mass-to charge, m/q, arrive at an entrance to the electrostatic trap as a respective focused packet of ions; 
 c. applying a substantially electrostatic trapping potential to at least a part of the electrode assembly, so as to generate an electrostatic trapping field within the trap for trapping the ions in a volume, V, of the electrostatic trap such that they undergo multiple isochronous oscillations along a longitudinal axis, z, of the electrostatic trap, wherein said oscillations are detected as a measured transient signal; 
 d. causing a perturbation in the electrostatic trapping field which results in at least a portion of the ions of each packet of ions to undergo a separation in oscillation phase of no more than 2π radians over a measurement time period, T m , the perturbation arising from at least one of the following: a distortion of the electrostatic trap geometry, a distortion of a part of the trapping potential, and application of an additional distortion potential to at least one part of the electrode assembly, 
 wherein the perturbation in the electrostatic trapping field is such that the detected transient decays from a maximum amplitude to no less than a specified percentage of the maximum amplitude over the measurement time period, T m . 
 
     
     
       9. The mass spectrometry method of  claim 8 , wherein the specified percentage is chosen from the group consisting of: 1%, 5%, 10%, 30% and 50%.